Ship bilge water normally contains substantial quantities of fuel oil and other hydrocarbons, as well as a substantial quantity of suspended solids. In the past, bilge water was merely pumped overboard, but due to recent environmental controls, it is required that oil sludge be separated from the bilge water before the water is discharged.
Various types of oil separating units have been used in the past to separate oil from bilge water. In most systems the separating unit is located on the suction side of the pump in order to minimize agitation of the bilge water and prevent emulsification of the oil. However, suction side systems cannot draw water to any substantial height and thus systems of this type are normally restricted to locations less than approximately 20 feet above the bilge sump.
Other oil separation systems, as used in the past, have located the separating unit on the discharge side of the pump and have pumped the bilge water to a gravity separator that operates under atmospheric pressure. In systems of this type, the separated oil has been discharged from the separator by gravity, and a second pump is frequently used to flow the water effluent from the gravity separator through a final separating stage. Attempts to utilize a single pump to discharge the heavy viscous oil from the separating unit located on the discharge side of the pump, have not been successful in that high pump pressures are required, and the high pressure tends to emulsify the oil and water during normal operation of the system. Thus, there has been a need for a system which employs a single pump operating at low pressure and which is still capable of pumping the heavy oil during the drain cycle.